1. Field of the Invention
The present invention relates to a skew adjustment circuit, a semiconductor device, and a skew calibration method, and in particular relates to the skew adjustment circuit of a clock signal used in a Clock and Data Recovery circuit (hereinafter referred to as a CDR circuit), and the semiconductor device including the CDR circuit.
2. Description of the Related Art
Along with speeding up of information communication apparatuses, a transfer rate of data (data rate) has been increased for transferring the data between semiconductor devices configuring the information communication apparatuses. For example, it is expected that the transfer rate per channel to transfer the data reaches 25 Gbps in 2016 and the transfer rate is further increased. The semiconductor devices are mounted on a circuit board such as a printed circuit board, and transmission and reception of the data are performed via wiring of the circuit board. In that case, the wiring of the circuit board functions as a transmission line for transferring the data; however, there is a transmission loss in the transmission line, and the transmission loss is increased according to an increase in the transfer rate of the data transmitted through the transmission line.
When the data of high transfer rate is attempted to be transferred by using the transmission line, due to the transmission loss in the transmission line and reflection, intersymbol interference is generated and a signal waveform is degraded. In particular, when the transmission line has a long distance, degradation of the signal waveform is increased, and transferring the data of high transfer rate becomes difficult.
As a semiconductor device for improving the degradation of the signal waveform, for example, there is a semiconductor device having a function of a signal conditioner. Such a semiconductor device includes, for example, an equalizer, and improvement of the waveform degradation is achieved by the equalizer. To improve the waveform degradation by the equalizer, it is required to convert the waveform transmitted through the transmission line into a digital signal. In serial communication, the data to be transferred and a synchronous clock signal for identifying the data are superimposed on each other to be transferred as an input signal.
The semiconductor device having the function of the signal conditioner includes a CDR circuit, and extracts the data (digital signal) from the input signal by the CDR circuit. The data extracted is provided to the equalizer, and the improvement of the waveform degradation is performed.
A technique relating to the CDR circuit is described in, for example, JP-2008-66879-A.